The present invention relates generally to ion beam deposition processes, and more specifically to an improved ion beam process for depositing diamond-like carbon coatings onto a variety of substrates.
Diamond-like carbon (DLC) coatings have become an area of intense research and experimentation. Diamond-like carbon generally consists of hydrocarbon structures that exhibit properties similar to those of diamonds. Diamond-like carbon exhibits a low coefficient of friction, high wear resistance, extreme hardness, extreme corrosion resistance and exceptional scratch resistance. DLC used as a hard coating has diverse applications in such areas as tribology, optics and electronics.
A particular advantage of DLC coatings is that they can be applied at relatively low temperatures, typically lower than 300.degree. C. Diamond coatings, while usually displaying physical properties superior to those of DLC coatings, are created at much higher temperatures, typically higher than the melting temperature of the substrate onto which a coating is desired to be placed.
Many different methods for depositing DLC films or coatings currently exist. Several elements are common among these methods. Typically, a DLC deposition apparatus consists, in its most basic form, of an ion gun or ion source, an evacuation or vacuum chamber, and an apparatus for holding a substrate to be coated.
Prior art methods for producing DLC films utilize as an ion source derivatives of a so-called Kaufman source. A Kaufman ion source incorporates high current metallic filaments (so-called hot-filament sources) mounted inside a vacuum vessel to ionize hydrocarbon gas molecules, such as methane (CH.sub.4), to form a carbon rich plasma which is accelerated by the beam of ions from the Kaufman source to strike a substrate to be coated with the DLC film or coating.
Unfortunately, conventional prior art methods for depositing DLC coatings using Kaufman sources have many drawbacks. The primary disadvantage is that metal vapor produced by hot filaments inside the vacuum chamber contaminates the carbon ions extracted from the plasma. Moreover, the hot filaments have very short lifetimes, typically only a few hours, depending on the gaseous mixture and pressure.
Additionally, these prior art ion deposition methods only enable the user to deposit DLC onto a small planar surface area of a substrate. This severely limits the usefulness, particularly the commercial usefulness, of diamond-like carbon coatings.
A very important limitation of prior art methods is the types of substrates onto which DLC coatings can be successfully deposited. Adhesion between DLC and many substrates, particularly metals, is typically extremely poor due to the inherent compressive stress of DLC. The DLC film will usually expand or contract when removed from the vacuum chamber and that expansion or contraction results in compressive stresses inside the film which can make it peel away from the substrate. Existing methods of deposition attempt to remedy this problem by introducing an intermediate layer, such as silicon (Si), between the substrate and the DLC. This is done by first depositing the intermediate layer onto the substrate and then depositing the DLC on top of the intermediate layer.
There is a need in the prior art not only for improved apparatus for depositing DLC coatings, but also for the effects of control parameters such as ion energy, power, gas composition and substrate temperature on DLC characteristics. Those characteristics include such properties as adhesion to substrate, friction and wear behavior, infrared (IR) transmission and electrical properties. To date, despite much experimentation, and despite improved apparatus such as is described as part of the present invention, ion beam deposition of DLC coatings having characteristic properties suitable for a desired application is, at best, a hit or miss process. By "at best" is meant that most often no successful DLC coating for a particular application can be achieved. By "hit or miss" is meant that, even when successful DLC coatings are achieved, they are not repeatable and control parameters for achieving a practical degree of repeatability are not well defined.
An example of an application for DLC coatings is as a coating for infrared window materials. Zinc selenide (ZnSe) and zinc sulfide (ZnS) are currently used as domes or windows in infrared sensor systems. These are excellent optical materials for applications throughout the infrared and visible regions of the spectrum. Unfortunately, these materials are mechanically soft and undergo significant degradation when subject to chemical attack, rain erosion and sand impact. Thus, the development of economical techniques to significantly improve the hardness of these materials without degrading the integrity of the specular transmittance is a great current interest. DLC coatings, if they can be successfully applied to ZnS and ZnSe surfaces, will be of great value.
Another example of an application for DLC coatings is as a coating for surfaces intended to be used in the ultrahigh vacuum of space. An example of such a surface is chemical-vapor deposited (CVD) fine-grain diamond coatings. Both the coefficients of friction (0.4 to 2.0) and the wear rate (10.sup.-4 mm.sup.3 /Nm) of CVD diamond films are considerably higher than in air or in dry nitrogen. The presence of an amorphous, nondiamond carbon layer on CVD diamond films decreases both friction and wear in ultrahigh vacuum, resulting in a low steady-state coefficient of friction (&lt;0.1) and a low wear rate (.ltoreq.10.sup.-6 mm.sup.3 /Nm) One method for producing such a layer on CVD diamond films is ion implantation. Ion implantation produces acceptable levels of friction and wear of CVD diamond films, but the depth of implantation is very shallow which may limit the tribological applications of ion implantation to light loads or short-term operations. The thickness range of DLC films can be 0.1 to 5 .mu.m, an order of magnitude greater than ion-implanted layers. Therefore, an amorphous DLC film coated on a CVD diamond film can enhance the tribological properties of such films, particularly for increasing the endurance of CVD diamond films.
Thus it is seen that there is a need for an improved method for successfully depositing diamond-like carbon coatings directly onto a variety of substrate materials, and for the control parameters necessary for successful application of DLC coatings onto specific substrates.
It is, therefore, a principal object of the present invention to provide an improved method for depositing diamond-like carbon coatings onto a variety of substrate materials, and to define the control parameters necessary for successful application of DLC coatings onto specific substrates.
It is a feature of the present invention that its ion beam source will not contaminate ions extracted from the ion plasma.
It is a feature of the present invention that the characteristics of the source plasma can be monitored and controlled so that a user can monitor and control on a real-time basis the resulting characteristics of the diamond-like carbon coating.
It is another feature of the present invention that its ion beam source has an extended lifetime and more efficiently transfers ions to a substrate.
It is an advantage of the present invention that it can deposit diamond-like carbon coatings over large and nonplanar substrate surface areas.